This invention concerns an auxiliary lock with an extensible device. The U.S. Pat. No. 4,662,665 titled "Auxiliary lock with an extensible device" acquired by the same applicant as the invention has a structure, wherein the length of the lock is changed by the pin 642 of the linking plate 64 selectably hooking in one of the two notches 211 or 212 in coordination with the adjustment of the shell 4 and the rotating shell 113. The U.S. Pat. No. 4,664,433 titled "Latch helical backset adjustment", discloses a lock wherein the length of the latch is changed by the bolt guide pin 100 inserting and turning in the flat end portions 128 set in the bolt assembly 36. Besides, the U.S. Pat. No. 4,752,090 titled "Auxiliary lock with an extensible device" acquired by the same applicant as this invention has a structure, wherein the length of a lock is changed by the pin assembly on the dead bolt engaging the helical track formed on the post, and the longitudinal shaft of the post in the connecting plate being rotated.